(1). Field of the Invention
The present invention relates to a liquid crystal display element, which is utilized for an optical shutter and the like, and a manufacturing method thereof.
(2). Description of the Prior Arts
A conventional and general liquid crystal display element, wherein a twisted nematic liquid crystal material (hereinafter referred to as xe2x80x9cTN liquid crystalxe2x80x9d) and an active matrix mode are combined, changes an alignment state of a liquid crystal molecule according to the applied voltage by holding the TN liquid crystal between two substrates with an electrode and applying voltage between the above-mentioned substrates. Light transmittance in a liquid crystal molecule changes according to an alignment state. Thus, a mode (a longitudinal electric field type) in which light transmittance is controlled by voltage is used for TN liquid crystal. A liquid crystal display element is composed of an active matrix substrate with a switching element for controlling whether voltage is applied or not, typically a thin film transistor hereinafter referred to as xe2x80x9cTFTxe2x80x9d), and a pixel electrode; an opposite substrate; a liquid crystal sealed between both substrates; and a polarizer disposed outside both substrates. An alignment state of a liquid crystal molecule is changed by an applied voltage between electrodes on both of the substrates, and thereby light transmittance is changed
However, in such a general liquid crystal display element, optical activity of transmitted light for an observer changes according to viewing angle, namely, an angle with a screen, and consequently brightness is affected. For instance, an image with a high contrast ratio is observed in the case of viewing from a front direction vertical to a screen with a white display, namely, a normal direction to the screen, while an image is observed dark in the case of viewing the screen from an oblique direction below the normal direction. When the screen is viewed from a much lower direction, the phenomenon of tone reversal, wherein brightness and darkness are reversed, occurs. When the screen is viewed from an upper oblique direction, an image is observed white. This phenomenon occurs for the reason that a direction, in which a liquid crystal molecule is set up, is determined in a display mode wherein optical activity is controlled by applying electric field to liquid crystal in a normal direction to a substrate and setting up a liquid crystal molecule along a direction of the electric field. Since a multitude of micro globes xe2x80x98spacerxe2x80x99 is scattered for obtaining the uniformity of cell thickness, the unevenness occurs easily on a screen, whereby the grade of display is badly affected.
Recently, a lateral electric field type has been noticed as a means of improving such a problem of viewing ankle. As described above, in a conventional liquid crystal display element, electric field is applied in a normal direction to a substrate and an alignment state of liquid crystal is controlled, while a lateral electric field type is a mode of controlling by an electric field in a lateral direction parallel with a substrate. This mode supplies a wide viewing angle in principle, and is considered as an idea for improving with a highest effect because of little change in color tone. Thus, although a viewing angle of a lateral electric field type liquid crystal display element is greatly wider than that of a longitudinal electric field type, the element has a wide area of shielding such as a common electrode, a source electrode and a switching element, therefore an aperture ratio of a pixel has no choice but to become lower than a conventional TN type. Consequently, a bad influence of many spacer particles, which is scattered for obtaining the cell thickness, on the grade of display is greater than a conventional TN mode.
In order to solve these problems and to obtain a fine grade of display, the following methods have been employed. a column-shaped spacer is formed in an area of shielding on an active matrix substrate with such a switching element as TFT; and a column-shaped spacer is formed in an area of shielding, typically a black matrix in a pixel part of a color filter.
In a conventional TN type, a structure, in which it is unnecessary to scatter spacer particles, has been proposed. For instance, a liquid crystal display element, wherein a spacer support is formed by joining together projections provided on both of an active matrix substrate and a substrate with a color filter, has been proposed in Japanese Unexamined Patent Publication No. 7-281295.
As regards a lateral electric field type, a liquid crystal display element using no spacer particles has been proposed in Japanese Unexamined Patent Publication No. 6-214244. In the liquid crystal display element, both of a common electrode and a pixel electrode (a source electrode) are formed while setting up vertically to a substrate, and both of the electrodes serve as a spacer (a support). Therefore, spacer particles are not necessary.
A conventional liquid crystal display element, wherein a polymer dispersed liquid crystal material (hereinafter referred to as xe2x80x98PDLCxe2x80x99) and an active matrix substrate with such a switching element as TFT are combined, has a constitution in which a liquid crystal layer, wherein a liquid crystal molecule is dispersed in a droplet or a network into polymer hereinafter referred to as xe2x80x98polymer matrixxe2x80x99) with a matrix structure, is held between two substrates with an electrode. A molecular structure of both is designed in advance so that a refractive index of a liquid crystal molecule aligned in an applied direction is equal to that of the above-mentioned polymer matrix under an application of voltage. Since refractive indexes of the liquid crystal molecule and the polymer matrix are equal under an application of voltage, an incident light into a PDLC layer becomes a transmitted light. On the other hand, the liquid crystal molecule is aligned in a disorderly direction under no application of voltage. Consequently, refractive indexes of the liquid crystal molecule and the polymer matrix are ordinarily unequal, and thereby an incident light into a PDLC layer becomes a scattered light. Thus, a transmitting state and a scattering state of an incident light are switched in PDLC by whether voltage is applied or not.
Also in a conventional polymer dispersed liquid crystal display element, a multitude of micro globes xe2x80x98spacerxe2x80x99 is scattered for obtaining the cell thickness to realize a fine grade of display. The spacer in the polymer dispersed liquid crystal display element is used for a projection display for enlarging and projecting on a screen an image of light projected from a lamp with a high luminance, and a reflection display for displaying an external light by utilizing a reflector provided inside the liquid crystal display element.
A dispersion density of spacers in a liquid crystal layer can be lowered in appearance by providing a spacer beforehand in an area of shielding on a substrate used for conventional TN type liquid crystal display element or lateral electric field type liquid crystal display element. However, on the contrary, the problem is a deterioration in the grade of display due to a change in temperature. This is described below referring to FIG. 12.
The problem is a low-temperature foaming during a change from room temperature to low temperature. When a liquid crystal display element 100 is left at a low temperature, the volume of a liquid crystal molecule contracts. Then, the cell thickness needs to contract, namely, thin in the thickness direction according to the volume contraction of the above-mentioned liquid crystal molecule. A low-temperature foaming 108 is a phenomenon wherein a vacuum foam is caused in a liquid crystal layer since the cell thickness can not follow such volume contraction of the liquid crystal molecule. When a spacer 105 formed on a substrate by patterning is firm, the low-temperature foaming 108 is caused easily since the contraction of the cell thickness according to the volume contraction of a liquid crystal 104 at a low temperature is hindered by the above-mentioned spacer 105. A spacer with a proper elasticity is preferable in terms of the follow-up of the cell thickness at such a low temperature and the maintenance of a fine grade of display having no unevenness on a screen. The problem is a display spottiness during a change from room temperature to high temperature. This results from an increase in the nonuniformity of the cell thickness due to a rise in temperature. A liquid crystal molecule sealed in a liquid crystal display element expands by heating. As a result of the expansion of the liquid crystal molecule due to a rise in temperature, the volume of the liquid crystal molecule increases inside the liquid crystal display element.
An increase in the volume causes a change in the gap between substrates, namely, a change in the cell thickness. Then, in a conventional liquid crystal display element, a column-shaped spacer can not follow an increase in the cell thickness. Consequently, the problem is that since a change in the cell thickness is controlled by the thermal expansion of the liquid crystal molecule, the nonuniformity of the cell thickness increases and the equality in the grade of display is lost.
Moreover, in the case of using a polymer dispersed liquid crystal display element, particularly, a projection display for enlarging and projecting on a screen an image of light from a light source projected onto a liquid crystal display element, a spacer is enlarged for maintaining the cell thickness. A PDLC projection display is in a light scattering mode (black display mode), namely, a normally black mode under no application of voltage. Then, when a conventional globular spacer without coloring is used, the scattering of light does not occur in an area wherein the spacer is scattered and light transmits trough the spacer. Consequently, a black luminance in a black display becomes higher in proportion to a dispersion density of the above-mentioned globular spacers, whereby a deterioration in contrast on a screen is caused
In order to restrain such a deterioration in contrast due to a rise in a black luminance resulting from the transmission through a spacer under no application of voltage, a spacer comprising a black colorant or a colorant for shielding is occasionally used. Although a deterioration in contrast is restrained by preventing the transmission of light with the spacer comprising a colorant, another problem is noticed. The equality of display in a high gray scale is important for maintaining a higher contrast. Consequently, it is necessary to improve the uniformity of the cell thickness. A dispersion density of the spacers needs to be raised for improving the uniformity of the cell thickness. When a dispersion density of the spacers is raised, the spacers touch each other during scattering and form a lump easily. Therefore, the unevenness occurs easily on a screen. Since the spacer is enlarged, the unevenness occurs easily under an application of voltage. Thus, a shape and a characteristic of the spacer have a problem in maintaining a fine grade of display in consideration of contrast and gray scale.
The problem is a reliability at storage temperature, namely, a reliability when left at a high temperature in a conventional combination of a spacer formed directly on a substrate and a polymer dispersed liquid crystal display element. In the above-mentioned polymer dispersed liquid crystal display element, the following problem is caused by an abrupt change in temperature. This is described below referring to FIG. 13.
FIG. 13(a) shows a state of a liquid crystal display element 200 in the case of normal temperature (such as 20xc2x0 C.). In FIG. 13(a), a spacer 206 comes under little pressure by glass substrates 201 and 202 since the size of the spacer 206 is equal to the gap.
FIG. 13(b) shows a state of a liquid crystal display element in the case of high temperature (such as 85xc2x0 C.). In this case, the volume of a polymer matrix 205 and a liquid crystal 204 expands by heating, and the internal pressure of a liquid crystal layer 209 increases. Because of not being fixed by a seal member 203 unlike the peripheral part, the middle part of the glass substrates 201 and 202 is transformed as shown in FIG. 13(b) under the influence of the increased internal pressure, and thereby the gap becomes wider at a nearer point to the middle part away from the peripheral part. As a result, the expanded liquid crystal 204 concentrates in the middle part.
In returning abruptly from a state of FIG. 13(b) to normal temperature, the liquid crystal layer 209 contracts. Then, the liquid crystal concentrating in the middle part can not completely return to an initial state, and most of the liquid crystal is left in the middle part. Consequently, as shown in FIG. 13(c), the gap between the glass substrates 201 and 202 is different at each point. Thus, in a conventional polymer dispersed liquid crystal display element, the variation in the thickness of a liquid crystal layer is caused by the expansion and contraction of the liquid crystal layer due to an abrupt change in temperature. As a result, the problem is a dramatic deterioration in the grade of display.
[The Problems in a Conventional Example]
The summary of the above-mentioned problems in a conventional example is as follows:
(1) In a TN mode liquid crystal display element, the grade of display is deteriorated in using at a low temperature, resulting from a low-temperature foaming due to an abrupt change in temperature from room temperature to low temperature.
(2) In a TN mode liquid crystal display element and a polymer dispersed liquid crystal display element, the grade of display is deteriorated in using at a high temperature, resulting from the nonuniformity of the cell thickness.
(3) When a polymer dispersed liquid crystal display element is left in a state of an abrupt change in temperature, the nonuniformity of the thickness of a liquid crystal layer is caused by the expansion and contraction of the liquid crystal layer, and thereby the grade of display is deteriorated
(4) Resulting from a rise in a dispersion density of the spacers and the disposition of the spacers, the unevenness occurs on a screen and the grade of display is deteriorated.
The purpose of the present invention, in view of the above-mentioned problems in the Prior Arts, is to improve a temperature characteristic of the grade of display resulting from a firm column-shaped spacer with little elasticity, and to provide a liquid crystal display element with little deterioration in the grade of display due to a change in temperature and a manufacturing method thereof
For further details, the purpose is to provide a liquid crystal display element with a fine grade of display and no low-temperature foaming in using at a low temperature, wherein the spacer can follow the contraction of a liquid crystal molecule due to an abrupt change in temperature from room temperature to low temperature.
The purpose is to provide a liquid crystal display element with a fine grade of display and the uniformity of the cell thickness in using at a high temperature.
The purpose is to provide a polymer dispersed liquid crystal display element with a high grade of display, wherein a low-temperature foaming and the nonuniformity of the cell thickness at a high temperature do not occur in a range of usable temperatures.
The purpose is to provide a polymer dispersed liquid crystal display element with a fine grade of display, wherein the nonuniformity of the thickness of a liquid crystal layer is not caused by the expansion and contraction of the liquid crystal layer when left in a state of an abrupt change in temperature.
The purpose is to provide a liquid crystal display element with a fine grade of display and no unevenness, which corresponds to high contrast and gray scale by forming a column-shaped spacer (which means nonglobular spacers) in advance in an area of shielding on an active matrix substrate or an opposite substrate.
In addition, the purpose is to provide a method of manufacturing the above-mentioned liquid crystal display element.
In order to achieve the above-mentioned purpose, the present invention comprises the following means.
The invention according to claim 1 of the present invention is characterized by a liquid crystal display element comprising a column-shaped spacer for maintaining a certain gap between the substrates, which is disposed between a pair of substrates; and a liquid crystal layer sealed between the substrates; wherein the above-mentioned spacer has an elasticity; and the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the above-mentioned liquid crystal layer is constantly approximately equal to atmospheric pressure with a change in temperature within a range of usable temperatures of the liquid crystal display element.
According to the above-mentioned constitution, the spacer can constantly be deformed elastically within a range of usable temperatures of the liquid crystal display element. Consequently, it is possible to prevent the low-temperature foaming phenomenon and the nonuniformity of the cell thickness at a high temperature, and to intend to improve the grade of display.
A dispersion density of spacers can be determined at a range having no unevenness on a screen by providing a proper elasticity for a spacer. The improvement of the grade of display can be intended from such a viewpoint.
A polymer dispersed liquid crystal layer as well as a TN type liquid crystal layer and an STN type liquid crystal layer may be used as a liquid crystal layer.
The invention according to claim 2 is characterized by a liquid crystal display element according to claim 1, wherein the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the above-mentioned liquid crystal layer is constantly approximately equal to atmospheric pressure; and each of the spacer and the internal pressure of the liquid crystal layer varies linearly.
As described above, when each of the spacer and the internal pressure of the liquid crystal layer varies linearly, the uniformity of the cell thickness is improved greatly.
The invention according to claim 3 is characterized by a liquid crystal display element comprising a column-shaped spacer for maintaining a certain gap between the substrates, which is disposed between a pair of substrates; and a liquid crystal layer sealed between the substrates; wherein the above-mentioned spacer has an elasticity; and the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the abovementioned liquid crystal layer is constantly approximately equal to atmospheric pressure with a change in temperature within a range from normal temperature to the highest usable temperature of the liquid crystal display element.
According to the above-mentioned constitution, it is possible to prevent the nonuniformity of the cell thickness at a high temperature, and to intend to improve the grade of display.
The invention according to claim 4 is characterized by a liquid crystal display element according to claim 3, wherein the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the above-mentioned liquid crystal layer is constantly approximately equal to atmospheric pressure; and each of the spacer and the internal pressure of the liquid crystal layer varies linearly
According to the above-mentioned constitution, the uniformity of the cell thickness at a high temperature is improved greatly.
The invention according to claim 5 is characterized by a polymer dispersed liquid crystal display element comprising a column-shaped spacer for maintaining a certain gap between the substrates, which is disposed between a pair of substrates; and a polymer dispersed liquid crystal layer composed of a liquid crystal and a polymer, which is sealed between the substrates; wherein the above-mentioned spacer has an elasticity; and the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the above-mentioned liquid crystal layer is constantly approximately equal to atmospheric pressure with a change in temperature within a range of storage temperatures of the liquid crystal display element.
According to the polymer dispersed liquid crystal display element having the above-mentioned constitution, the spacer can constantly be deformed elastically within a range of storage temperatures. Consequently, it is possible to prevent the deformation of the gap between the substrates (the nonuniformity of the thickness of the liquid crystal layer) from being caused by an abrupt change in temperature and to maintain a certain cell thickness, and to intend to improve the grade of display.
The invention according to claim 6 is characterized by a polymer dispersed liquid crystal display element according to claim 5, wherein the sum of a repulsion force of the above-mentioned spacer and an internal pressure of the above-mentioned liquid crystal layer is constantly approximately equal to atmospheric pressure; and each of the spacer and the internal pressure of the liquid crystal layer varies linearly.
As described above, when each of the spacer and the internal pressure of the liquid crystal layer varies linearly, the uniformity of the cell thickness is improved greatly.
The invention according to any one of claim 7 and 8 is characterized in that the above-mentioned spacer is formed directly on at least one of the above-mentioned pair of substrates by a patterning process.
According to the above-mentioned constitution, it is possible to obtain a spacer for which a desirable elasticity is provided in consideration of the size and the dispersion density of the spacer
The invention according to any one of claims 9 and 10 is characterized in that the above-mentioned spacer comprises a kind of material or a composite material consisting of two or more kinds of materials among a polystyrene-based polymer compound, an acryl-based polymer compound, a polyester-based polymer compound, a silicon -based polymer compound, a polycarbonate-based polymer compound, a polysiloxane-based polymer compound, a polyethylene-based polymer compound, and a polyurethane-based polymer compound.
The invention according to any one of claims 11 and 12 is characterized in that the above-mentioned spacer is formed in a shape with a thin tip from a substrate toward the other substrate.
According to the above-mentioned constitution, an elastic spacer with a desirable elasticity can be obtained.
The invention according to any one of claims 13 and 14 is characterized in that the above-mentioned spacer has a structure, in which the whole or a part of the spacer is hollow, between the above-mentioned pair of substrates.
According to the above-mentioned constitution, an elastic spacer with a desirable elasticity can be obtained
The invention according to any one of claims 15 and 16 is characterized in that the above-mentioned spacer has a distribution of length in a normal direction between the above-mentioned pair of substrates.
The invention according to any one of claims 17 and 18 is characterized in that the above-mentioned spacer has a distribution comprising an average and a variance in length in a normal direction between the above-mentioned pair of substrates; and the variance is in a range of approximately 3 to 6% of the average.
According to the above-mentioned constitution, the most proper elasticity can be provided for a spacer in consideration of a distribution of the height of the spacer
The invention according to any one of claims 19 and 20 is characterized in that a dispersion density of the above-mentioned spacer is in a range of approximately 5 to 2000/mm2.
According to the above-mentioned constitution, the most proper elasticity can be provided for a spacer in consideration of a distribution of the dispersion density of the spacer.
The invention according to any one of claims 21 and 22 is characterized in that the above-mentioned spacer is formed in an area of shielding on at least one of the above-mentioned pair of substrates.
According to the above-mentioned constitution, an aperture ratio is improved.
The invention according to any one of claims 23 and 24 is characterized in that an internal pressure of the above-mentioned liquid crystal layer is in a range of approximately 0 to 0.9 kgf/cm2.
When the spacer is in a state of elastic deformation, the internal pressure of the liquid crystal layer is lower than atmospheric pressure. Accordingly, a state of elastic deformation in the spacer can be determined by the internal pressure of the liquid crystal layer.
The invention according to claim 25 is characterized by a method of manufacturing a liquid crystal display element comprising a column-shaped spacer for maintaining a certain gap between the substrates, which is disposed between a pair of substrates; and a liquid crystal layer sealed between the substrates; comprising the steps of executing a treatment for providing an elasticity with a predetermined value for the spacers after forming a plurality of spacers on at least one of the pair of substrates; forming the liquid crystal layer sealed between the pair of substrates with the spacers; and pressing the gap between the substrates equally at a pressure value of approximately 0.1 to 1.0 kgf/cm2 simultaneously with or after the step of forming a liquid crystal layer.
The reason for controlling the force of pressing the substrates is that elastic deformation can not be caused in the spacer with a lower pressure value than 0.1 kgf/cm2, while plastic deformation is caused in the spacer with a higher pressure value than 1.0 kgf/cm2.